Electron discharge device



Sept. 13, 1960 R. A. CRAIG ET AL ELECTRON DISCHARGE DEVICE 2 Sheets-Sheet 1 Filed June 24, 1957 5 2 on mm INVENTORS ROBERT A CRAIG KURT E. ZUBLIN,

BY THEIR ATTORNEY Sept. 13, 1960 R. A. CRAIG ET AL ELECTRON DISCHARGE DEVICE 2 Sheets-Sheet 2 Filed June 24, 1957 FIG.3.

S E m L D L E F ELECTRIC x x MAGNETIC FIELD LINES m TYPE MODE ELECTRIC INVENTORS ROBERT A. CRAIG KURT E. ZUBLIN,

WQ/Yfi THE AT RNEY.

FIELD LINES TM" TYPE MODE MAGNETIC United States Patent Q ELECTRON DISCHARGE DEVICE Robert A. Craig, Palo Alto, and Kurt E. Zublin, Menio Park, Califi, assignors to General Electric Company, a corporation of New York Filed June '24, 1957, Ser. No. 667,428

12 Claims. (Cl. SIS-3.5)

This invention relates to ultrahigh-frequency electron discharge devices and more particularly to such devices of the traveling-wave tube type. It also relates, in a more specific aspect, to coupling devices for use in such tubes.

Traveling-wave tubes in one form include an electromagnetic wave translating element commonly referred to as a slow-wave structure, through which an electromagnetic Wave is propagated at a velocity substantially less than the velocity of light, and an electron beam which is directed in energy interacting relation with the high frequency electric field associated with the wave propagated by the slow-wave structure. Input means is included for introducing electromagnetic wave energy at one end of the slow-wave structure and output means is provide at the opposite end of the slow-wave structure for coupling out electromagnetic wave energy.

Slow-wave structures may take various forms. One preferred form having certain advantages at high power levels consists of a tubular waveguide containing a series of spaced parallel transverse plates or bailies, the plates having apertures therein adapted for passage of an electron beam therethrough. The baffles furnish periodic loading of the waveguide sufficient to reduce the wave velocity as required. Such a waveguide structure may be considered as the electrical equivalent of a series of coupled cavity resonators or filter sections. As is well known to those skilled in the art, a resonant cavity of itself is nonpropagating, but a series of coupled cavity resonators allows a wave of electromagnetic energy to propagate by the following mechanism. The excitation of one cavity is transferred through the coupling means to the next cavity. The second cavity is thus excited and the energy in it is transferred to the third cavity and so on. Thus, if the coupling is repeated in an identical or nearly identical manner for a series of cavities, the final result can be considered as a type of transmission line or waveguide filter. A coupler constructed according to the present invention is adapted to couple energy to a resonant cavity or to a slow wave structure consisting of a cascade of mutually coupled cavities.

A problem in achieving efficient operation in traveling wave tubes employing a slow-wave structure of the coupled cavity resonator type arises from the fact that two or more wave modes of excitation may occur in the slow wave structure over the same frequency band, each mode having a difierent field configuration and difierent propagation characteristics. For single electron beam traveling wave tube operation, the TM type mode is preferred, and it is therefore desirable, in coupling wave energy into and out of the slow-wave structure, to couple selectively to the TM type mode and to inhibit transfer of energy to other modes. Selective coupling to the desired mode is particularly advantageous in matching impedances when the waveguide bafiles are so designed as to permit transfer of energy between waveguide sections by means of negative mutual inductance coupling.

Accordingly, it is a principal object of the present invention to provide an improved traveling-wave tube in- Patented Sept. 13, 1960 cluding coupling means enabling selective coupling of wave energy to and from a selected excitation mode of a slow-wave structure.

Another object is to provide a traveling wave tube including an improved coupler for transferring energy to or from a cavity resonator, which coupler is relatively simple and easy to manufacture from a mechanical standpoint, and capable of operating eflficiently with relatively wide dimensional tolerances.

Another object of the present invention is to provide a traveling wave tube having improved coupling means for coupling electromagnetic wave energy into and out of a slow-wave structure with a low reflection coefficient over a wide frequency band.

Another object is to provide an improved coupler of simplified and relatively inexpensive construction for selectively transferring electromagnetic wave energy between a resonant cavity and a single coaxial transmission line.

Still another object of the present invention is to provide an improved traveling-wave tube amplifier having substantial overall gain, and requiring no internal attenuating means.

These and other objects and advantages of the invention will become apparent from the following description taken in conjunction with the accompanying drawings, and the scope of the invention will be defined in the appended claims.

In the attainment of the foregoing objects according to one aspect of the invention, we provide a traveling wave tube including a cavity resonator type slow-Wave structure and including improved coupling means adapted to be disposed in adjacent coaxial relations with the slow- Wave structure for transferring energy to or from the slow-wave structure. The coupler comprises a coaxial line segment arranged to be T-connected at its midpoint to a coaxial input or output line, and coupling antennae at the ends of the coaxial line segment. The antennae, which may be of the probe or loop type, are positioned to be in electrically symmetrical relation with the midpoint of the line segment, and to extend in energy transferring relation into physically symmetrical points in the electromagnetic field in the adjacent cavity resonator. Wave energy is thus selectively and efficiently transferred energy between the single coaxial line T-connected to its midpoint of the coaxial line segment, and the axially symmetrical field which characterizes the TM type mode in the cavity resonator while minimizing energy transfer to or from non-symmetrical modes.

For a detailed explanation of our invention, reference is made to the accompanying drawings in which like reference characters are used to designate like elements throughout, and in which:

Figure 1 is an axial view of a traveling wave tube constructed in accordance with the present invention;

Figure 2 is an enlarged isometric View of a portion of the slow-Wave structure of the tube of Figure 1;

Figure 3 is a cross sectional View of the slow wave structure of the tube of Figure 1, showing the fiield configuration of the TM type mode therein.

Figure 4 'is a view similar to Figure 3, but showing the field configuration of the TM type mode.

Figure 5 is an exploded isometric'view of a coupler constructed according to the present invention; and

Figure 6 is an enlarged fragmentary axial sectional view of another traveling-wave tube constructed in accordance with the present invention.

Figure 1 shows a traveling-wave tube constructed according to the present invention. The tube 11 includes an electron gun 13 consisting of a cathode 15, a cathode heater 17 connected to a suitable energizing source (not 'secured to bulb 23 and a cup-shaped electron collector anode 29 located at the right-hand end of the tube 11.

Anode 29 dissipates residual energy of the electrons in beam 25 and may be cooled, if desired, in any suitable manner as, for example, by a fluid in cooling coil 30.

The slow-wave structure 27 illustrated consists of a series of periodic waveguide sections or cavity resonators 31 stacked in coaxial adjacent relation and coupled together through axial apertures. Each section 31 is of the so-called cloverleaf type, a full description of which is given in a publication by one of us, Robert A. Craig, entitled, Study of Periodic Structures for High Voltage "Traveling-Wave Tube Operation, Technical Report #36, dated November 2, 1954, and published by Stanford University. As best shown in Figure 2, each section 31 includes a cylindrical portion 33.having its walls indented at 90 intervals to form wedge shaped radial finger 35 providing a clover-leaf-shaped cross section. The portion 33 is closed at each end by a transverse circular plate 39 which is centrally apertured as at 43, and serves to separate each section 31 from the adjacent section. The

' fingers 35 in each succeeding periodic section 31 are progressively angularly displaced 45 about the axis of the sections. a

The central aperture 43 in each of the plates 39 permits passage of the axial electron beam 25, and each plate 39 also has eight equiangularly spaced radial slots 45, between which the fingers 35 are situated and through which energy may be coupled from one section 31 to the next. The sections 31 serve in the aggregate as the electrical equivalent of a loaded transmission line, reducing the velocity of electromagnetic waves propagated therethrough to a value substantially below that in free space.

As shown in Figure 1, electromagnetic wave energy is coupled into the tube 11 from a single coaxial input line 47, and coupled out of the tube to a single coaxial output line 49, by couplers 55 and 56 respectively, the details of which will be described hereinbelow.

Coupler 55 is situated between drift tube 28 and the left-hand end of slow-wave structure 27, while coupler 56 is disposed between slow-wave structure 27 and collector anode 29. The bulb 23, drift tube 28, input coupler 55, slow-wave structure 27, output coupler 56, and collector anode 29 are joined together to provide an evacuated envelope for the tube 11. Focusing of the electron beam 25 throughout its travel along the'axis of slow wave structure 27 may be obtained by a unidirectional axial magnetic field produced by a solenoid 57.

Suitable direct current operating potentials for the tube may be obtained, as shown in Figure 1, from a power source 58, having a grounded positive terminal 59 to which the collector anode 29 is connected. Cathode 15 is connected to the negative terminal of source 58, while focusing electrode 19 may also be maintained at cathode potential, for example by direct connection to cathode 15 as shown. Accelerating anode 21 is connected to an adjustable tap 61 on source 58 and maintained thereby'at a potential positive with respect to cathode 15.

One ditficulty in obtaining eificient operation of traveling. wave tubes of the type above described is that at certain frequencies .the waveguide sections comprising the slow-wave structure 27 can be excited not only in the desired TM type mode, but also in difierent undesired modes such as the TM type mode. It is thus possible to have the cavities excited in an undesired mode simultaneously with, or instead of, a desired mode. By

reference to Figures 3 and 4 of the drawing, it will be seen that the TM type mode is characterized by an electric field pattern which is symmetrical about the cavity resonator axis, a desirable property for single electron beam traveling wave tube operation, while the TM type mode is characterized byan electric field pattern which is not angularly about the axis, and hence undesirable for single electron beam operation. The problem of obtaining eficient operation under such circumstances is overcome, in accordance with our invention, by the provision of improved coupling means for coupling selectively to electromagnetic wave energy in the TM type mode in. the slow wave structure 27. One embodiment of a coupler constructed in accordance with the invention, namely coupler 55 of Figure 1, is shown in detail in the exploded view of Figure 5.

As shown in Figure 5, the coupler 55 includes two disks 80, 82 of electrically conductive material adapted to be secured together in abutting end-to-end relation. The disks have central apertures 84, 86, and the mating surfaces of the disks have concentric confronting annular grooves 88, of semicircular cross section, which when the disks are joined mate to form a closed circular passage 92. Concentrically supported within the passage is a parti-circular inner conductor 94. To the midpoint of conductor 94 is connected, through an aperture 96 in disk 80, the center conductor 98 of the coaxial 'input transmission line 47. Center conductor 98 may be oriented at any desired angle relative to inner conductor 94, and aperture 98 may be located in the cylindrical exterior surface of disc 80 if desired, The outer conductor of the line .47 is electrically connected to disk 80, and hence in efiect .to the outer conductor of the coaxial line formed bythe passage 92 and conductor 94 within the coupler. Various impedance ratios between line 47 and the two branches of the line 92, 94 may be used, but a two-to-one ratio has been found to give good results. For example, line 47 may have a characteristic impedance of 50 ohms, while each of the parallel lines formed by the branches ofinner conductor 94 may have an impedance of 100 ohms.

Opposite the ends of the inner conductor 94 are apertures 102, 104 in disk 82 opening onto the opposite face thereof. Through the apertures 102, 104 extend conductors 106, 108, which are connected to the respective ends of the inner conductor 94. Extending axially from the face of disc 82 remote from disc 80 is a peripheral flange rabbeted at its outer end to receive a cover such as the adjacent transverse plate 39 of the first section 31 of the slow-wave structure 27. The length of flange 110 is not critical, but is preferably relatively small to minimize the length of conductors 106, 108. Plate 39 together with the disk 82 and flange 110 form a cavity 114 between which and the passage 92 energy is transferred through the antennae formed by conductors 106, 108.. Conductors 106, 108 may be electrically connected to plate 39 to serve as coupling loops,

or they may terminate in spaced relation with plate 39 so as to serve as coupling probes. Also provided in cavity 114 are a pair of diametrically spaced radial vanes 116,, the purpose of which will be described below.

When assembled in the tube the coupler is placed adjacent one end of the slow-wave structure 27, for example, between the lefthand end of the slow-wave structure and the drift tube 28 as shown in Figure 1. The coupler is arranged so that the central apertures 84, 86 therein are coaxial with the tube 11 to permit unob- V structed passage of the electron beam, and so that the symmetrically located in cavity 114, it will be appreciated that the coupler selectively couples to the symmetrical field which, as illus rated in Figure 3, characterizes the TM type mode in the cavity. The portion of passage 92 from which inner conductor 94 is omitted acts as a cylindrical Waveguide whose dimensions are too small to support a significant electromagnetic field at the tube operating frequency. The vanes 116 serve to perturb the electromagnetic fields in cavity 114 to facilitate coupling to the adjacent section of the slow wave structure 27, and also serve as impedance matching devices to match the impedances of the coupler 55 and the slow wave structure 27. Impedance matching of the coupler to the slow-wave structure may also be accomplished in several other ways, such as by varying the impedance of the two conductors 1G6, 108 or by changing the size of each conductor 196, 108 as it enters into the cavity 114, or by varying the diametrical spacing between conductors 106, 108 in order to change the ratio of electric to magnetic field strength at the points of insertion, or by varying the size of the vanes 116 to vary the amount of coupling area that the conductors 106, 108 efiectively intercept.

As shown in Figure 1, wave energy in the TM type mode is selectively coupled out of the slow-wave structure by coupler 56, which is identical in all respects to coupler 55, and is situated between collector anode 29 and the final section of the slow wave structure 27. Coupler 56 extracts energy from the slow wave structure and transfers it to output coaxial line 49.

Additionally, as shown in Figure 6, a traveling Wave tube may be provided having two couplers 132, 134, each similar to couplers 55 and 56, arranged back-to-back at an intermediate point in the slow wave structure to remove undesired wave energy from the slow wave structure. The central apertures of the couplers serve as a drift tube 135 through which the electron beam modulated in the first portion 136 of the slow wave structure may pass to the second portion 138 of the slow wave structure. If desired, the length of this drift tube may be conveniently varied merely by changing the axial dimension of the couplers. The electromagnetic wave energy which has propagated along the first portion of the slow wave structure is coupled out to a suitable attenuating load (not shown) by coupler 132, While undesired energy reflected from the second portion of the slow wave structure is coupled out by coupler 134. By effectively extracting undesired wave energy from the tube, the coupler arrangement shown in Figure 6 provides a convenient means of minimizing spurious oscillations, thus avoiding the limitation on gain heretofore imposed by such spurious oscillations without requiring the introduction of special energy-absorbing material in the slow-wave structure. Thus a relatively high-gain traveling wave tube is provided requiring no internal attenuation, and afiording relatively high efficiency of operation.

Thus it may be seen that the present invention provides an ultra-high frequency electron discharge device including an improved low loss means of effectively transferring energy between a coaxial line and a selected symmetrical mode of excitation. The improved coupling means is simplified from a mechanical standpoint and thus lends itself to manufacture at reduced cost. Though particularly adapted for convenient assembly with a waveguide-type slow wave structure, the coupler may likewise be employed with a cavity resonator, waveguide, or other structure capable of being permeated by electromagnetic wave energy excited in a mode characterized by a field pattern symmetrical about an axis. Accordingly, the expression waveguide as used hereinafter in the claims is intended to embrace not only wave propagating structure of the hollow pipe type but also cavity resonators as well.

It will be appreciated by those skilled in the art that the invention may be carried out in various ways and may take various forms and embodiments other than 6 those illustrative embodiments heretofore described. It is to be understood therefore, that the scope of the invention is not limited by the details of the foregoing description, but will be defined in the following claims.

What we claim as new and desire to secure by Letters Patent of the United States is:

1. A coupler for transferring electromagnetic energy between a coaxial line and a waveguide, said coupler comprising a housing including a coaxial line segment having a hollow outer conductor element and an inner conductor element, a cylindrical flange extending from one face of said housing and forming with said housing a cavity adapted to be disposed in adjacent relation with said waveguide and coupled thereto thereby to define a cavity resonator for transfer of electromagnetic energy between said waveguide and said antennae, antennae connected to opposite ends of said inner conductor of said coaxial line segment and protruding longitudinally from said housing at points located for disposition opposite symmetrical points in the field of a selected excitation mode in said waveguide, and a coaxial line having a hollow outer conductor and an inner conductor, said inner and outer conductors electrically connected to corresponding conductor elements of said coaxial line segment midway between said antennae for coupling electromagnetic energy between said waveguide and said coaxial line when said cavity resonator is excited.

2. A coupler for transferring electromagnetic energy between a coaxial line and a hollow cylindrical waveguide having a transverse bafile adjacent one end thereof comprising an annular housing adapted to be disposed adjacent the one end of the waveguide and containing a coaxial line segment having a hollow outer conductor element and an inner conductor element, a cylindrical flange extending from one face of said housing in coaxial relation with said waveguide and forming a cavity resonator with said housing and the transverse battle of the Waveguide, antemiae connected to opposite ends of said inner conductor element of said coaxial line segment and protruding longitudinally from said housing at points located opposite symmetrical points in a transverse plane of said cavity, and a coaxial line having a hollow outer conductor and inner conductor electrically T-connected to corresponding conductor elements of said coaxial line segment midway between said antennae to provide coupling of electromagnetic energy between said waveguide and said coaxial line when said cavity resonator is excited.

3. A coupler for transferring electromagnetic energy selectively between a coaxial line and a selected excitation mode of a Waveguide, said coupler comprising a pair of plates of electrically conductive material secured together in face-to-face relation to form a housing, one of said plates having a pair of apertures located for disposition opposite symmetrical points in the field of the selected mode, confronting grooves in the mating faces of said plates forming a passage in the housing extending between said apertures, a center conductor concentrically supported in said passage and forming therewith a coaxial line segment extending between said apertures, a coaxial line electrically connected to the midpoint of said coaxial line segment, an outstanding flange on said one plate forming with said one plate a cavity resonator adapted to be excited in said selected mode, and antennae connected to points on said coaxial line segment symmetrical with the midpoint of said segment, said antennae extending through said apertures for insertion into symmetrical points in the field of the selected wave guide mode whereby the selected electromagnetic energy is coupled between said waveguide and said coaxial line.

4. A coupler for transferring energy selectively between a coaxial line and a selected excitation mode of a waveguide, said coupler comprising a pair of plates .of electrically conductive material secured together in faceto-face relation to form a housing, one of said plates fronting grooves of semi-circular cross section in the mating faces of said plates forming an annular passage in the housing extending between said apertures, a center conductor concentrically supported in said passage and forming therewith a coaxial line segment extending between said'apertures, said housing having an opening opposite the midpoint of said coaxial line segment, a single coaxial line electrically connected to the midpoint of said coaxial line segment at said opening, and antennae connected to points on said coaxial line segment opposite said apertures and equally spaced from the midpoint of said segment, said antennae extending through said apertures for insertion into symmetrical points in the field of the selected mode in said Waveguiding hollow device whereby electromagnetic energy is coupled between said coaxial line and said waveguide.

-5. An electron discharge device comprising a waveguide adapted to be excited with electromagnetic wave energy in a selected mode, means for directing an electron beam through said waveguide in energy-coupling relation with said selected excitation mode, a coaxial line having a hollow outer conductor and an inner conductor,

and coupling means for transferring electromagnetic energy between said waveguide and said coaxial line, said coupling means including a cavity resonator and a coaxial line segment having a hollow outer conductor element and an inner conductor element mounted with its ends disposed opposite points in said waveguide symmetrical with the axis of said waveguide, said coaxial line segment being T-connected at its midpoint to said coaxial line in such a manner that corresponding conductor elements of said coaxial line and coaxial line segment are electrically connected, said cavity resonator being coaxially positioned adjacent said waveguide for transfer of electromagnetic energy therewith and antennae extending into said cavity resonator at points symmetrically to the axis electromagnetic wave energy in a selected mode, means for directing an electron beam through said waveguide and the opening in said baffle in energy-coupling relation with said selected excitation mode, a coaxial line, and coupling means for transferring electromagnetic energy between said waveguide and said coaxial line, said coupling means comprising a housing disposed adjacent I said bafiie, a flange extending between said housing and said baflie and forming with said housing a cavity separated from said waveguide by said baflie, said housing having 'a pair of apertures located opposite symmetrical points in said cavity, a coaxial line segment in said housing extending between said apertures, said coaxial line segment being T-connected at its midpoint to said coaxial line, and antennae connected to points on said coaxial line segment equally spaced from the midpoint of said segment, said antennae extending through said apertures into symmetrical points in said cavity whereby electromagnetic energy is coupled between said coaxial line and said waveguide.

7.- In combination, a waveguide having a transverse bafiie adjacent one end, said bafiie having openings through which electromagnetic Wave energy may be transferred to and from said waveguide, a coaxial line for 7 energy transfer with said waveguide, and an energy transferring coupler between the coaxial line and said waveguide, said coupler comprisinga housing disposed adjacent said baflle, a flange extending between said housing and said baflle and forming with said housing a cavity separated from said waveguide by said baffle, said housing having a pair of apertures located opposite symmetrical points in said cavity, a coaxial line segment in said housing extending between said apertures, said coaxial line segment being T-connected at its midpoint to said coaxial line, and antennae connected to points on said coaxial line segment equally spaced from the mid point of said segment, said antennae extending through said apertures into symmetrical points in said cavity whereby electromagnetic energy is coupled between said Waveguide and said coaxial line.

8. 'In a traveling wave tube, a slow-wave structure comprising a hollow cylindrical waveguide having a trans verse baflie said baifle having a central aperture to allow the passage of an electron hem through said slow wave structure and having additional openings through which electromagnetic wave energy may be transferred, a coaxial line having a hollow outer conductor and an inner conductor for energy transfer with said slow wave structure, and an energy transferring coupler between the coaxial line and said slow-wave structure comprising a coaxial line segment having a hollow outer conductor element and an inner conductor element mounted with its ends disposed opposite points on said baffle symmetrical with the axis of said slow-wave structure, said coaxial line being shunt-connected to said coaxial line segment in such-a manner that corresponding elements of said coaxial line and said coaxial line segment are electrically connected, and antennae connected to the ends of the inner conductor element of said coaxial line segment whereby electromagnetic energy is coupled between said waveguide and said coaxial line.

9. In a traveling wave tube, a slow wave structure comprising a hollow cylindrical waveguide containing a series of spaced transverse baflles forming with said waveguide a seriesof cavity resonators, said baflies having central apertures to allow the passage of an electron beam axially through said waveguide and having each a plurality of additional openings for transferring electromagnetic wave energy from one cavity resonator to the next, a coaxial line for energy transfer with one of the cavity resonators, and an energy transferring coupler between the coaxial line and said one cavity resonator, said coupler comprising a housing disposed adjacent one of the baffles of said one cavity resonator and including a coaxial line segment, a cylindrical flange extending between said housing and said one cavity resonator and forming with said housing a cavity closed at its end and separated from said one cavity resonator by said one baflle, said housing having a pair of apertures located opposite symmetrical points in said cavity, a throughbore in said housing coaxial with said slow wave structure for passing said electron beam, a coaxial line segment in said housing extending between said apertures, said coaxial line being T-connected to the midpoint of said coaxial line segment, and antennae connected to points on said coaxial line segment equally spaced from the midpoint of said segment, said antennae extending through said apertures into symmetrical points in said cavity whereby electromagnetic energy is coupled between said waveguide and said coaxial line.

10. A traveling wave tube including a slow wave structure comprising a hollow cylindrical waveguide containing a series of spaced transverse baflies forming with said waveguide a series of cavity resonators, said baffles having central apertures to allow the passage of an electron beam axially through said waveguide and having each a plurality of radial slots for transferring electromagnetic wave energy from one cavity resonator to the next, a coaxial line for energy transfer with one of the cavity resonators, an energy transferring coupler between the coaxial line and said one cavity resonator comprising a pair of plates of electrically conductive material secured together in face-to-face relation to form a housing and disposed adjacent one of the baflies of said one cavity resonator, a cylindrical flange on said housing forming with said housing a cavity closed at its end and separated from said one cavity resonator by said one bafiie, one of said plates having a pair of apertures located opposite symmetrical points in said cavity, a central throughbore in said housing for passing said electron beam, confronting annular grooves of semi-circular cross section in the mating faces of said plates forming an annular passage in the housing concentric with said throughbore and extending between said apertures, a center conductor concentrically supported in said passage and forming therewith a coaxial line segment extending between said apertures, said coaxial line being T-connected to the midpoint of said coaxial line segment, and antennae connected to points on said coaxial line segment equally spaced from the midpoint of said segment, said antennae extending through said apertures into symmetrical points in said cavity whereby electromagnetic energy is coupled between said waveguide and said coaxial line.

11. A traveling Wave tube comprising a slow wave structure including a hollow Waveguide comprising a series of coaxial Waveguide sections of four-leaf-clovershaped cross section each rotatively displaced relative to the next by 45 and separated by transverse baffles forming with said waveguide sections a series of cavity resonators, said bafiles having central apertures to allow the passage of an electron beam axially through said waveguide and having each a plurality of radial slots for transferring electromagnetic wave energy from one cavity resonator to the next, a coaxial line for energy transfer with one of the cavity resonators, an energy transferring coupler between the coaxial line and said one cavity resonator comprising a pair of plates of electrically conductive material secured together in face-to-face relation to form a housing and disposed adjacent one of the baflies of said one cavity resonator, a cylindrical flange on said housing forming with said housing a cavity closed at its end and separated from said one cavity resonator by said one 'baflle, one of said plates having a pair of apertures located opposite symmetrical points in said cavity, a central throughbore in said housing for passing said electron beam, confronting annular grooves of semicircular cross section in the mating faces of said plates forming an annular passage in the housing concentric with said throughbore and extending between said apertures, a center conductor concentrically supported in said passage and forming therewith a coaxial line segment extending between said apertures, said coaxial line being T-connected to the midpoint of said coaxial line segment, and antennae connected to points on said coaxial line segment equally spaced from the midpoint of said segment, said antennae extending through said apertures into symmetrical points in said cavity.

12. In a traveling Wave tube, first and second coaxial slow-wave structure sections each including a cylindrical Waveguide, means for directing an electron beam axially through said first and second sections, means for introducing electromagnetic wave energy into said first section in energy coupling relation With said electron beam, means for extracting electromagnetic energy from said second section, and first and second couplers disposed between said first and second sections of slow-Wave structure, said couplers including a housing having a throughbore coaxial with said slow-wave structure for passing said electron beam, said first coupler including a first coaxial line segment arranged with its ends extending into points in said first slow wave structure section symmetrical with the axis thereof, a first output coaxial line T-connected to the midpoint of said first coaxial line segment for extracting undesired wave energy from said first slow wave structure section and transmitting said wave energy external to said tube, said second coupler including a second coaxial line segment arranged with its ends extending into points in said second slow-Wave structure section symmetrical with the axis thereof, and a second output coaxial line T-connected to the midpoint of said second coaxial line segment for extracting undesired wave energy from said second slow-wave structure section and transmitting said wave energy external to said tube.

References Cited in the file of this patent UNITED STATES PATENTS 2,106,770 Southworth et al Feb. 1, 1938 2,402,184 Samuel June 18, 1946 2,412,446 DeWalt Dec. 10, 1946 2,681,951 Warnecke et al June 22, 1954 2,712,614 Field July 5, 1955 2,742,612 Cohn Apr. 17, 1956 2,814,756 Kenmoku Nov. 26, 1957 2,850,671 Dench Sept. 2, 1958 2,869,020 Sensiper Jan. 13, 1959 OTHER REFERENCES Currie et al.: The Cascade Backward-Wave Amplifier, in Proceedings of the I.R.E., November 1955; page 1619. 

